1. Field of the Invention
The present invention relates to a waveform processing circuit for use, for example, in a reproducing circuit of a photomagnetic disc system.
2. Description of the Prior Art
The waveform processing circuit will be described in connection with a reproducing circuit of a photomagnetic disc system, as an example.
A photomagnetic disc recording and reproducing system has a laser beam generator for generating a converged laser beam having a diameter of about 1 micrometer on a disc. The disc has a laminated film which shows a photomagnetic effect.
In the recording mode, a relatively strong pulsating laser beam (high level beam) carrying information is impinged on the disc to increase the temperature of the impinged spot on the film. The spot on the film where the temperature is increased frees the coercive force so that the spot is magnetized in a certain direction dependent upon a magnetic field produced around the disc by a magnetic field generator, thereby recording the information.
In the reproducing mode, a relatively weak and constant linearly-polarized laser beam (low level beam) is impinged on the disc and the reflected beam is detected by a detector. By the Kerr magnetooptical effect on the film, the reflected beam shows the inclination of the polarization face due to the change of magnetized direction of the film. When the inclination is viewed through an analyzer, a pulsating signal representing the change of the inclination can be detected by a light detector. The pulsating signal produced from the light detector will be the reproduced signal.
Generally, a recording of information on the photomagnetic disc is effected on a track aligned with a plurality of sectors (FIG. 2), each sector including a head portion where the address data and other similar data are stored with a high frequency signal in a predetermined format, and a data portion, following the head portion, which is utilized for recording and reproducing the desired data at a high frequency. During the recording mode, the head portion is impinged with a low level beam to read out (reproduce) the address data and other data stored therein thereby locating the sector on the disc. Then, the data portion following the detected head portion is impinged with the high level beam carrying information to record data therein. During the reproducing mode, the head portion is reproduced by the low level beam in the same manner to locate the sector being impinged with the beam, and then, the data portion following the head portion is impinged with the low level beam to reproduce the recorded data. Accordingly, even in the recording mode, the reproducing operation is carried out for reading the address data in the head portion.
The reproduced signal Sa from the light detector, such as shown in FIG. 2, contains a high frequency component signal (VD-VL) or (VP-VL) carrying wanted data and a low frequency component signal VL caused by the conversion between the optical signal and the electric signal. The head portion contains a high frequency signal (VD-VL). The data portion contains, in the recording mode, a high frequency signal (VP-VL), and in the reproducing mode, a high frequency signal (VD-VL). The signal (VP-VL) is greater than the signal (VD-VL). In order to remove the low frequency component VL, a high pass filter such as defined by a coupling capacitor Ca and a resistor Ra as shown in FIG. 1 is used. The reproduced signal as obtained from the light detector is a current signal, which is converted to a voltage signal by a suitable current-voltage converter. The reproduced signal Sa in the voltage form as produced from the current-voltage converter is applied to the high pass filter, and the filter produces a filtered signal Sb which is also shown in FIG. 2.
When the input signal Sa is the recording mode is applied to the high pass filter, the high pass filter produces an output signal Sb such that the low frequency signal VL is cut off, and the high frequency signal (VD-VL) passes therethrough. However, due to the transient phenomena of the high voltage signal VP, the output signal Vo will be deformed greatly. More specifically, as shown in FIG. 2, the output signal Sb gradually decreases after the receipt of the leading edge of the high voltage signal VP and gradually increases after the receipt of the trailing edge of the high voltage signal VP. In this case, since the head portion, as included in the gradual increasing line, may be located outside the dynamic range, the head portion may not be properly read in the subsequent stage by the signal detector. Although it is possible to widen the dynamic range to include the head portion therein, the signal becomes poor particularly when a signal is detected at the comparator.